Engine oil
You don't change this - just keep adding more (either with the fuel, or in the oil reservoir if the engine has oil injection)
Gearbox oil
There are two screws on the side of the gearbox (at the bottom of the leg). Take both of them out to let the old oil drain out (try to control where it goes - it smells disgusting).
Check that the old oil hasn't gone milky. If it has, you've lost a seal and water is getting into the gearbox (get this fixed - ignoring it will rapidly prove expensive)
Inject fresh oil into bottom hole (you can buy a pump to do this, or get your oil in a squeezy tube with the right size of nozzle) until it starts to overflow out the top hole. Replace the top hole plug to airlock the oil in, before removing the nozzle and plugging the filling hole.
where is the drain oil plug on a 2010 9.9 mercury outboard motor
Web site never did answer the question.
whereiscrank case oil drain plug on 1998 mercury 9.9 4 stroke
what type oil to use on 4stroke 2.5mercury outboard
Take it to a marine repair shop or google it up, the answer is there
I have a 1999 Mercury 2stroke 5HP outboard and the owners manual states 50:1 Gas/Oil mixture.
A 175hp Mercury outboard requires a 50:1 fuel to oil ratio.
2%
If manufactured after 1965, an 80 hp Mercury outboard requires a 50:1 fuel, to 2 cycle oil ratio.
The 1973 model, 7.5 hp Mercury outboard, requires 6.8 oz. of gear oil.
A 4hp Mercury outboard requires a 50:1 fuel to 2 cycle oil mix.
The 4.5 HP Mercury Outboard requires 50 to 1 fuel, to 2 cycle oil ratio. A mix of six gallons of gallons of fuel to one pint of oil is a good mix for the Outboard.